Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> writes: > On 11.07.2016 15:12, Xunlei Pang wrote: >> On 2016/07/11 at 17:54, Wanpeng Li wrote: >>> Hi Konstantin, Xunlei, >>> 2016-07-11 16:42 GMT+08:00 Xunlei Pang <xpang@xxxxxxxxxx>: >>>> On 2016/07/11 at 16:22, Xunlei Pang wrote: >>>>> On 2016/07/11 at 15:25, Wanpeng Li wrote: >>>>>> 2016-06-16 20:57 GMT+08:00 Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>: >>>>>>> Hierarchy could be already throttled at this point. Throttled next >>>>>>> buddy could trigger null pointer dereference in pick_next_task_fair(). >>>>>> There is cfs_rq->next check in pick_next_entity(), so how can null >>>>>> pointer dereference happen? >>>>> I guess it's the following code leading to a NULL se returned: >>>> s/NULL/empty-entity cfs_rq se/ >>>> >>>>> pick_next_entity(): >>>>> if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) >>> ^^^^^^^^^^^^^ >>> I think this will return false. >> >> With the wrong throttled_hierarchy(), I think this can happen. But after we have the >> corrected throttled_hierarchy() patch, I can't see how it is possible. >> >> dequeue_task_fair(): >> if (task_sleep && parent_entity(se)) >> set_next_buddy(parent_entity(se)); >> >> How does dequeue_task_fair() with DEQUEUE_SLEEP set(true task_sleep) happen to a throttled hierarchy? >> IOW, a task belongs to a throttled hierarchy is running? >> >> Maybe Konstantin knows the reason. > > This function (dequeue_task_fair) check throttling but at point it could skip several > levels and announce as next buddy actually throttled entry. > Probably this bug hadn't happened but this's really hard to prove that this is impossible. > ->set_curr_task(), PI-boost or some tricky migration in balancer could break this easily. sched_setscheduler can call put_prev_task, which then can cause a throttle outside of __schedule(), then the task blocks normally and deactivate_task(DEQUEUE_SLEEP) happens and you lose. The obvious way to avoid these would be to somehow change put_prev so that it only does throttles in the schedule() path (which is what we /want/), which would probably involve adding a parameter to put_prev for just this. -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html